A Unified Control Strategy for Three-phase Inverter in Distributed Generation

ABSTRACT:  

This paper presents a unified control strategy that permits both islanded and grid-tied operation of three-phase inverter in distributed generation (DG), with no need for switching between two corresponding controllers or critical islanding detection. The proposed control method composes of an inner inductor current loop, and a novel voltage loop in the synchronous reference frame (SRF).

The inverter is regulated as a current source just by the inner inductor current loop in grid-tied operation, and the voltage controller is automatically activated to regulate the load voltage upon the circumstance of islanding. Furthermore, the waveforms of the grid current in grid-tied mode and the load voltage in islanding mode are distorted under nonlinear local load with the conventional method.

And this matter is addressed by proposing a unified load current feed forward in the paper. also, the paper presents the detailed analysis and the parameter design of the control strategy. Finally, the influence of the proposed control method is validated by the simulation and experimental results.

KEYWORDS:

  1. Distributed generation
  2. Three-phase inverter
  3. Islanding
  4. Unified control
  5. Seamless transfer
  6. Load current

SOFTWARE: MATLAB/SIMULINK

CIRCUIT DIAGRAM:

 Fig. 1 Schematic diagram of the distributed generzation based on the proposed control strategy.

 EXPECTED SIMULATION RESULTS:

Fig. 2 Bode plot of the transfer function from load current to grid current with

and without the load current feedforward, when DG operates in grid-tied

mode.

Fig. 3 Simulation waveforms of load voltage vCa, grid current iga and inductor

current iLa when DG is in grid-tied mode under condition of the step down of

the grid current reference from 9A to 5A with: (a) conventional voltage mode

control, and (b) proposed unified control strategy.

Fig. 4 Simulation waveforms of load voltage vCa, grid current iga and inductor current iLa when DG is transferred from grid-tied mode to islanded mode with: (a) conventional hybrid voltage and current mode control, and (b) proposed unified control strategy.

 CONCLUSION:

 A unified control method is proposed for three-phase inverter in DG to operate in both isolate and grid-tied mode, with no need for switching between two different control architectures or critical islanding detection. A novel voltage controller is presented. It is inactivated in grid-tied mode, and the DG operates as a current source with fast dynamic performance.

Upon the utility outage, the voltage controller can automatically be activated to regulate the load voltage. Moreover, a novel load current feedforward is proposed, and it can improve the waveform quality of both the grid current in grid-tied mode and the load voltage in isolate mode. The proposed unified control method is verified by the simulation and experimental results.

 REFERENCES:

[1] R. C. Dugan and T. E. McDermott, “Distributed generation,” IEEE Ind. Appl. Mag., vol. 8, no. 2, pp. 19-25, Mar./Apr. 2002.

[2] R. H. Lasseter, “Microgrids and distributed generation,” J. Energy Eng., vol. 133, no. 3, pp. 144-149, Sep. 2007.

[3] C. Mozina, “Impact of Green Power Distributed Generation,” IEEE Ind. Appl. Mag., vol. 16, no. 4, pp. 55-62, Jul./Aug. 2010.

[4] IEEE Recommended Practice for Utility Interface of Photovoltaic(PV) Systems, IEEE Standard 929-2000, 2000.

[5] IEEE Standard for Interconnecting Distributed Resources with Electric Power Systems, IEEE Standard 1547-2003, 2003.

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